1. Field of the Invention
The present invention generally relates to semiconductor transistors, and more particularly, to an improved structure of (and method for forming) an inverse-T gated metal oxide semiconductor field effect transistor.
2. Description of the Related Art
Fabrication processes for metal oxide semiconductor field-effect transistors (xe2x80x9cMOSFETxe2x80x9d) devices are well known. Gate structures for MOSFETs are generally manufactured by placing an undoped polycrystalline silicon (xe2x80x9cpolysiliconxe2x80x9d) layer over a relatively thin insulator (xe2x80x9cgate oxidexe2x80x9d) layer. The gate oxide sits on a substrate having a well region. The polysilicon layer and the oxide layer are then patterned to form a gate conductor over the well region and the structure is subjected to implanted impurities to make selective regions conductive. Such implantation serves both to dope the gate conductor and to form lightly-doped regions (xe2x80x9cLDDxe2x80x9d) in the silicon substrate.
If the dopant species used is n-type, then the resulting MOSFET is typically an NMOS (xe2x80x9cn-channelxe2x80x9d) transistor device. Conversely, if the dopant species is p-type, then the resulting MOSFET is typically a PMOS (xe2x80x9cp-channelxe2x80x9d) transistor device. Integrated circuits typically use either n-channel devices exclusively, p-channel devices exclusively, or a combination of both on a single substrate. The combination of a n-channel device and a p-channel device on a single substrate is termed a complementary MOS (xe2x80x9cCMOSxe2x80x9d) device. In such structures, one of the active regions, typically the region in which the p-channel device is to be formed, is covered with a masking layer. N-type dopants are implanted into the n-channel devices.
After the first doping process, insulating sidewall spacers are formed on the sidewalls of the gate structure. A second implant dose is then forwarded into the gate structure and the silicon substrate. The second implant is done at a higher implant energy and dose than the first and creates source/drain regions within the silicon substrate. The gate conductor is preferably used to self-align the impurities implanted into the substrate to form the source and drain regions. The process is then repeated for the p-channel transistor, except now p-type dopants are implanted and the n-channel transistors are protected with a mask.
It has been found advantageous to utilize a gate conductor that has the shape of an inverted xe2x80x9cTxe2x80x9d when viewed in a cross-section. Specifically, an inverse-T gate has a thick center section bordered by wings that are thinner. Such a structure allows a small portion of impurities to pass through the thinner outer portions (wings) of the inverted-T structure into the substrate while simultaneously blocking such impurities from the thicker main part of the gate conductor. Therefore, with an inverse-T gate structure, the LDD regions can be simultaneously formed with the heavily doped source/drain regions in a single doping process (as opposed to the two-stage doping process discussed above).
Various methods of fabricating MOSFETs with inverse-T gate structures have been tried previously. For example, inverse-T shaped gates in MOSFETs are disclosed in U.S. Pat. No. 5,654,218 and U.S. Pat. No. 5,241,203, which are hereby incorporated by reference. U.S. Pat. No. 5,654,218 discloses a process using an isotropic etch to undercut a first sacrificial layer to form the wing-type structures forming the gate conductor. Control of this undercutting and resulting dimensions are inadequate in view of the smaller dimension of such devices being used today. The U.S. Pat. No. 5,241,203 teaches of a timed etch to control thickness of the wing-structures forming the gate structure. Again, control of etching of these wing-structures is inadequate with today""s ultra-small devices. Thickness variations of the wing structures result in varying concentration and depth of the lightly-doped-drain (LDD) and halo implants.
In other words, inverse-T gate structures are conventionally manufactured by etching the gate itself or etching masks that form the gate. Such processes are inherently difficult to control because they are heavily dependent upon slight variations in the etching/undercutting processes. Therefore, any slight variation in pressure, temperature, time, chemical concentration, etc. will cause an inconsistent material removal which will vary the thickness of the wings of the inverse-T gate structure. The amount of doping which passes through the wings of the inverse-T gate structure and reaches the underlying substrate is highly dependent upon the thickness of the wings. When the manufacturing process does not consistently produced wings having a uniform thickness, the doping of the substrate regions below the wings becomes inconsistent. This leads to non-uniform device performance and increases the defect rate.
As inverse-T gate conductors become smaller and smaller with advancing technology, these variations in the etching/undercutting processes produce inconsistencies beyond acceptable manufacturing tolerances. Therefore, there is a need for a new system/method of manufacturing inverse-T gate structures that does not rely upon an etching/undercutting process. Further, there is a need for a process which consistently manufactures inverse-T gate structures with the same size dimensions to ensure uniform doping of the LDD regions. The invention described below provides such a method/system.
In view of the foregoing and other problems of the conventional methods of making inverse-T gate structures, the present invention has been devised, and it is an object of the present invention to provide a structure and method for making inverse-T gate structures in MOSFETs that uses a damascene process and avoids controlled etches for wing thicknesses.
The invention describes a field effect transistor having a substrate with a well region, a source region and a drain region. Also included in the invention is a gate oxide above the well region and a gate conductor above the gate oxide. The gate conductor has an inner inverse-T structure and an outer conductive material different than the inner inverse-T structure. Further, the outer conductive material covers the top and sides of the inverse-T structure. The outer conductive material is preferably tungsten. Also, the outer conductive material and the inverse-T structure are the same length along the gate oxide. The outer conductive material has an inverse-U shape that matches an outline of the inverse-T shape of the inner inverse-T structure. The inverse-T structure contains a center portion with a thickness greater than outer wing portions. The wing portions are a different conductive material than the center portion.
The invention further includes a gate oxide which has a center section that is thinner than the outer regions of the gate conductor. The center section of the gate oxide includes a higher nitrogen content than the outer regions of the gate oxide. The gate conductor has an inverse-T structure. The outer regions of the gate oxide are positioned under the wing portions.
Another object of the present invention is to form a field effect transistor using a method that deposits the conductor layer on the underlying layer, patterns a mask over the conductor layer, forms sidewall spacers within an opening in the mask and deposits another conductor between the sidewall spacers within the opening. The first and second conductors are inverse-T gate conductors, The method removes the mask, removes portions of the first conductor layer outside the sidewall spacers, and removes the sidewall spacers.
The invention also forms an oxide layer on a substrate to form the underlying layer. Further, the invention implants an impurity into the substrate to form source and drain regions in the substrate adjacent the inverse-T gate conductor. The invention implants a single impurity and simultaneously forms lightly-doped-drain and source regions in the substrate under the wing portions and more heavily doped drain and source regions in the substrate adjacent the wing portions. Further, implanting the impurity into the substrate can be used to form a halo implant in the substrate under the inverse-T gate conductor. In addition, the inverse-T gate conductor has a center portion with a thickness greater than outer wing portions. The implanting process forms the halo implant in the substrate under the wing portions. The invention implants the impurity into the well region of the substrate through the opening in the mask. In the invention, the thickness of the conductor layer determines the thickness of the wing portions.
Further, the invention forms a sacrificial layer within the opening, sacrificial spacers along walls of the opening, removes the first sacrificial layer, deposits a conductor within the opening, removes the mask and removes the sacrificial spacers.
In addition, the present invention forms an oxide layer in the presence of nitrogen such that the oxide layer has outer portions under the sacrificial spacers that are thicker than a center section of the oxide layer, deposits a conductor within the opening, removes the mask and removes the sacrificial spacers.
The invention provides a new system/method of manufacturing inverse-T gate structures that does not rely upon an etching/undercutting process. Further, the invention consistently manufactures inverse-T gate structures with the same size dimensions to ensure uniform doping of the LDD and halo regions.